1. Field of the Invention
This invention relates to a particle measuring method and apparatus, and more particularly to a particle measuring method and apparatus in which a laser beam is used to illuminate a detection zone in a medium, laser light scattered by particles in the medium is detected by a photosensor and measurement is effected by measuring particle properties based on output signals from the photosensor.
2. Description of the Prior Art
Apparatuses are known for optically measuring particles in a liquid or gaseous medium by utilizing scattered light. With such apparatuses, particles are measured by using light receiving elements to evaluate the scattering of the laser light that impinges on the liquid or gas concerned (See for example Japanese Laid open Publication No. 64-18043). However, because the light scattered by the particles has a very low intensity, it is necessary to use means to ensure that scattered light impinging on the light receiving element is of a sufficiently high intensity.
This usually consists of controlling the intensity or polarization of the incident light, or the direction in which the scattered light is received. Conventional methods include utilizing all of the laser light that has been randomly polarized as the incident light or, in the case of linearly polarized laser light, arranging the polarization to be perpendicular to the scattering plane for the measurement.
When the linearly polarized incident light oscillates perpendicularly with respect to a plane obtaining between the axial orientation of the incident light and the axial orientation along which the scattered light is received ( the scattering plane ), it is referred to as S-polarized light, and when the oscillation of the incident light is parallel to the scattering plane, it is referred to as P-polarized light.
Particularly in the case of apparatuses for determining particle size by measuring the intensity of laterally scattered light from particles in a liquid that are irradiated with a beam of laser light, linearly polarized laser light is picked up as S-polarized light, when particles are 0.1 microns or smaller, because the contribution of the P-polarized incident component from laterally scattered light is negligibly small compared to the S-polarized incident component.
FIG. 2a illustrates the relationship between scattered light intensity and particle size in accordance with one example in which monochromatic light is used. Up to a particle size of around 0.3 microns the scattered light intensity is a single-valued function of particle size, but above that size this function does not obtain. More specifically, below 0.3 microns precise measurement is possible because there is a 1:1 correspondence between the scattered light intensity and particle size, but above that size this becomes impossible. This means that 0.3 microns is the effective limit of measurable particle size.
In this regard, conventional methods that have been used include eliminating multiple values by using a light source for producing polychromatic light, or expanding the single-valued zone to the size of the larger particles by measuring the intensity of forwardly scattered light. However, in order to cover a wide range of particle sizes including 0.1 microns and below, the use of lateral scattering is indispensable to prevent stray light in the measurement cell, and also in order to utilize the convergency of the laser light it has become necessary to expand the range of measurable particle sizes on the basis of the lateral scattering method that utilizes the monochromaticity of the laser beam.
When the incident laser light is set to be P-polarized light, a single-valued function obtains even when the particle size is above 0.3 microns, but then the dynamic range of the photosensor becomes a problem.
The photosensor usually has a usable dynamic range which limits the range of scattered light intensities. As such, even if the range of particle sizes in which a single-valued function obtains is expanded, it becomes impossible to process the signals.
On the other hand, one arrangement is used to reduce the background light due to a medium in the scattered light measurement zone by providing an optical mask in the light detecting system. An automatic alignment is also possible whereby in accordance with the output of the photosensor the mask is located at the optimum position in relation to the focal depth of the imaging lens or scattered light measurement zone. Such a mask alignment system is disclosed in Japanese Laid-open Patent Application No. 64-18043 above mentioned. In such an apparatus, when the amount of scattered light is reduced so that it matches the dynamic range of the photosensor, there occurs the risk that it will prevent accurate operation of the automatic mask alignment system described above.
Further, it is known that a photon counting method is preferably used to detect a very weak intensity of light scattered from a single particle smaller than 0.1 microns in diameter ( see for example U.S. Pat. No. 4,830,494), Since this photon counting method employ a digital processing, stable signals are obtained against outer electrical disturbance, thus enabling precise particle measurement. The photon counting method is, however, impractical for measuring particles whose diameter ranges above 0.1 microns, for example, because a too strong intensity of scattered light is received by the photosensor.